1. Field of the Invention
The invention relates to the field of optical measurement of electric voltages.
2. Discussion of Background
A fiber-optic voltage sensor of the type mentioned at the beginning is known, for example from EP 0 316 635 B1. The sensor principle is based on the fact that a strain, induced by an electric field, in a piezoelectric sensor body is transmitted to a fiber, and the change in length of the latter is detected interferometrically. Quartz is frequently used as the piezoelectric material. Various types of glass fiber interferometers can be used for the interferometric measurement, such as a Mach-Zehnder, a Fabry-Perot or a double-mode fiber interferometer, for example. It is known from EP 0 529 339 A1 to connect in series in the manner of a tandem interferometer two double-mode fiber interferometers separated by a monomode fiber which is not susceptible to interference. The tandem interferometer is operated in transmission or, in accordance with EP 0 569 700 A2, in reflection. A variant with a polarimetric sensor fiber instead of a double-mode sensor fiber is disclosed in DE 196 05 717 C1. It is also known from DE 41 15 370 A1 to detect the change in length of a double-mode fiber with the aid of the Guoy effect.
Various methods are employed to compensate the temperature dependence of the fiber-optic voltage sensor. It is proposed in EP 0 433 824 B1 for the piezoelectric sensor and demodulation elements to be of identical design and to be operated at the same temperature. For this purpose, DE 42 29 449 specifies special quartz sections and fiber fastenings, for which the temperature response of the piezoelectric effect can be eliminated. The German Patent Application with the file number 197 01 221.3, which was not published before the priority date, shows that it is possible in a double-mode fiber interferometer to determine the sensor temperature from the interference contrast and to compensate its influence on the sensor signal.
In accordance with EP 0 316 619 B1, it is possible to implement selective measurement of a single E-field component by means of special crystal classes, orientations and shapes of the piezoelectric body, as well as by the type of the fiber fastening. In EP 0 316 635 B1 cited firstly above, it is disclosed, furthermore, that a voltage sensor which integrates the local E-field can be approximated by lining up such direction-sensitive sensor elements with dielectric spacers.
A range of variants for integrating a fiber-optic voltage transformer in a gas-insulated switchgear (GIS) are published in the prior art. DE 40 25 911 A1 shows for a one-phase GIS that the high voltage can be determined reliably by local field measurements using several sensor elements which are arranged on the inside of the GIS enclosure distributed over the circumference. By contrast, in the German Patent Specification with the file number 196 04 908.3, which was not published before the priority date, a voltage measurement is implemented in a three-phase GIS by bridging the entire high voltage in each phase by means of a quartz cylinder together with its fiber winding. The three voltage sensors are completely immersed in the SF.sub.6 overpressure atmosphere of the GIS in order to insulate the very high E-field strengths. In addition, an insulating tube is provided for protecting the quartz cylinders and sensor fibers against chemically aggressive gases. Moreover, DE 195 48 467 A1 shows a suspension, which is not susceptible mechanically to interference, of the fiber-optic voltage sensor, in which axial forces act antisymmetrically at both ends on the end faces of the quartz cylinder, with the result that the fiber strain averaged over the cylinder length vanishes. All the documents named above are hereby intended to apply in their full scope as a constituent part of the disclosure of the present invention.
To date, no fiber-optic voltage sensor has been specified which would be suitable for measuring high voltage outdoors. On the one hand, local field measurements are completely inadequate outdoors, since the E-field distributions vary strongly depending on the weather and the circuit state of neighboring outdoor switches. On the other hand, it is very expensive to bridge the entire potential difference using an approximating voltage sensor several meters long in accordance with EP 0 316 635 B1. An adequate integration accuracy would require very many sensor elements. Nevertheless, the total voltage would be divided between the sensor and intermediate elements, with the result that a variable dielectric response of the intermediate elements would falsify the voltage drop across the sensor elements, and thus the measuring accuracy.
A flexible suspension of thyristor power converters for oil or gas platforms or for protection against earthquakes is known from DE 3 010 294 C2. In this case, several power converters are fastened to a yoke next to one another via insulators and, if necessary, clamped to the ground via insulators in series to form damping elements. The design is clearly not intended for high voltages.
Finally, there is a widespread prior art relating to current sensors with an isolated sensor head. These include, in particular, magneto-optic current sensors having a glass block sensor head or a sensor fiber (see, for example, the German Patent Applications, which were not published before the priority date, with the file numbers 196 21 654.0 and 197 03 128.5), but also hybrid optical current sensors with an electronic sensor head at high voltage potential and optical power supply and signal transmission.